This invention relates to a positioning stage system and a position measuring method usable in an exposure apparatus, for example, for manufacturing of semiconductor devices, for example. According to another aspect, the invention is concerned with an exposure apparatus having such a positioning stage system.
With further miniaturization and increasing density of a semiconductor chip due to enlargement of integration of semiconductor devices, a very narrower linewidth has been desired. For a semiconductor device of 1 GDRAM, for example, it should have a linewidth of 0.18 micron. As regards the registration of printed patterns, it should be 80 nm for a semiconductor device of 256 MDRAM and 60 nm for a semiconductor device of 1 GDRAM.
Exposure apparatuses for lithographically transferring a pattern formed on a mask to a substrate such as a wafer, are also required to provide high precision and good linewidth precision. While i-line or KrF laser are used as exposure light, to prevent degradation of resolution due to diffraction, a proximity exposure apparatus based on step-and-repeat process and using X-rays of shorter wavelength has been proposed.
A proximity exposure apparatus performs an exposure process while keeping a wafer, held on a wafer stage, close at a small gap of about 10-50 microns to a mask. The wafer stage is moved in X and Y directions to move stepwise exposure shots on the wafer to an exposure region, opposed to the mask, sequentially. Alignment measurement for the mask and wafer is performed by using an alignment optical system, and then an X-ray beam (exposure light) is projected to the mask whereby the pattern formed on the mask is transferred to the wafer.
In such exposure apparatus, it is important to position a mask and a wafer precisely. A high precision positioning stage system is therefore necessary. As a measuring system for a wafer positioning stage, a laser interferometer capable of measuring the movement amount of a movable member with high precision may be used.
Such a laser interferometer generally includes a laser oscillator (laser head) for emitting laser light, a measuring mirror mounted on a positioning stage to be measured, a reference mirror, which provides a measurement reference, an optical system having a polarization beam splitter, for example, for distributing the laser light from the laser oscillator to the measuring mirror and the reference mirror, and a photodetector. The laser light emitted from the laser oscillator is separated by the polarization beam splitter, so that a portion of the laser light passes through the polarization beam splitter and it is projected on the measuring mirror mounted on the positioning stage, which is the target to be measured. The remaining portion of the laser light is reflected by the polarization beam splitter toward the reference mirror. Light reflected by this reference mirror goes through the polarization beam splitter, while light reflected by the measuring mirror is reflected by the polarization beam splitter, whereby both are directed to and detected by the same photodetector. These laser lights to be detected by the photodetector interfere with each other to produce interference fringes. The photodetector is used to count the fringes, on the basis of which the distance between the reference mirror and the measuring mirror is detected. In accordance with the result, the position or movement amount of the positioning stage (target of measurement) having the measuring mirror mounted thereon can be determined.
Here, the reference mirror provides a measurement reference, and preferably it should be fixed to a base member which is integrally connected with measuring optical components. Also, it should be disposed close, as much as possible, to a member (e.g., mask) on which a reference should inherently be defined. This reduces the effect of thermal expansion between them. For example, in a projection exposure apparatus using g-line or i-line as exposure light, a reference mirror for a wafer X-Y stage for moving the wafer stepwise to the exposure station may be fixed to a base member of a lens barrel (Japanese Laid-Open Patent Application, Laid-Open No. 163354/1994).
In an X-ray exposure apparatus of the proximity type wherein unit-magnification exposure is performed while holding a mask and a wafer close to each other, a measurement reference for a wafer X-Y stage may preferably be provided by a mask itself or a mask supporting member close to the mask.
In X-ray exposure apparatuses of the proximity type, generally, a mask, which should provide a reference, is mounted for small movement in a rotational direction or tilt direction relative to an X-Y measurement beam. For example, a mask supporting member for holding the mask is structured to have freedom in a rotational direction along the mask surface so that it can absorb a mask manufacturing error or conveyance error. Alternatively, it is structured to have freedom in a tilt direction along the mask surface so that it can absorb a wedge component of the mask. In such a structure, if the reference mirror is provided on the mask or mask supporting member, the mask or mask supporting member, which defines the measurement reference, is displaceable in the rotational direction or tilt direction relative to the X-Y measurement beam, such that accurate measurement is not assured. Further, movement of the mask produces other components in X and Y directions which, although they are small because they are based on a cosine error, may lead to the result that the projected measuring beam does not come back to the photodetector. In that case, the wafer stage positioning is not attainable. The reference mirror may be mounted on any immovable member disposed outside the mask holding member in an attempt to avoiding the above problem. However, then the distance to the mask become long, and there arises the problem of a non-negligible error due to thermal expansion.
It is accordingly an object of the present invention to provide a positioning stage system and a position measuring method, by which high-precision measurement and high-precision positioning are attainable constantly regardless of the displacement of a measurement reference for a laser interferometer.
It is another object of the present invention to provide an exposure apparatus having such a stage positioning system, or a device manufacturing method using such an exposure apparatus.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.